The operation of an internal combustion engine, such as an automotive vehicle engine, creates many gaseous exhaust byproducts. Included in these byproducts are pollutants such as unburned hydrocarbon, carbon dioxide, carbon monoxide, and various nitrogen-oxygen compounds, which are known in the art as NOx. NOx, as it is found in the exhaust of an internal combustion engine, primarily consists of nitrogen oxide (NO) and nitrogen dioxide (NO2). NOx is a pollutant that can cause many problems including, but not limited to, eye, nose, and throat irritation. NOx may also combine with water to produce acids that are a component of acid precipitation. Modern automotive vehicles are fitted with an engine exhaust treatment system to alleviate these problems and comply with environmental regulations.
Often, an engine exhaust treatment system will include an adsorbent for taking up unburned hydrocarbon from the exhaust. The adsorbent is disposed in the exhaust piping that leads away from the engine. The adsorbent attracts the unburned hydrocarbon and accumulates it in the interstitial spaces on the surfaces of the adsorbent. The adsorbent physically occludes the unburned hydrocarbon until the oxidation function of a subsequent catalytic converter has reached its light-off temperature, at which point the adsorbent also attains an elevated temperature at which it releases the unburned hydrocarbon for transport to the downstream catalytic converter. The catalytic converter thereafter oxidizes and converts the unburned hydrocarbon in the exhaust gas into non-polluting gases.
A common type of catalytic converter is known in the art as a lean NOx trap (LNT). As the name implies, the LNT converts NOx to a nonpolluting gas. The LNT, however, functions at a high efficiency and removes the greatest amount of NOx only after it has reached a critical light-off temperature, above about 200 degrees Celsius. In the case of an automobile started in a location having low ambient temperatures, it can take several minutes for the LNT to reach this critical light-off temperature, during which time the NOx disadvantageously escapes to the atmosphere. In this case, over 50% of all NOx tailpipe emissions produced during a drive cycle can occur during the time it takes the LNT to reach 200 degrees Celsius.
It would be desirable to provide apparatus for reducing the amount of NOx escaping to the atmosphere during cold operation of an internal combustion engine, when the LNT or other NOx removal technology is below about 200 degrees Celsius.